In the process of electrolyzing sodium chloride in diaphragm cells, gaseous hydrogen simultaneously generating with the electrolysis due to, for instance, partial exfoliation of fragile asbestic diaphragm contaminates gaseous chlorine to form a chlorine-hydrogen detonating gas and the gas explodes by a certain cause. Almost all the causes of such an explosion of the chlorine-hydrogen detonating gas are due to the discharge of static electricity accumulated within the tower installed for drying the gaseous chlorine from the cells.
Since in the process of electrolyzing sodium chloride, simultaneously with the generation of chlorine, gaseous hydrogen also generates on the other hand, risk of explosion is always present, and many accidents of explosion have been reported from the factories of electrolysis of sodium chloride in several districts.
That the cause of these explosions is the discharge of accumulated static electricity is supported by the repeatedly detected electrical charge of .sym.100 to 1000 V at the top of the tower made of polyvinyl chloride for drying gaseous chlorine and .crclbar.5000 V in the lower space of the tower where sulfuric acid flows down during the survey on static electrical charge in the tower in operation.
Seeing that such an electrical potential is present, it is presumed that the risk of spark discharge is always present and an accident of explosion is easily caused in the case where such a detonating gas within the explosion limits is co-existent with the risk.
As a means for preventing this kind of accidents, the following means have been hitherto adopted actually:
(1) An apparatus for continuously determining hydrogen in the industrial aqueous chlorine is provided, and the operation of the process is strictly administered to maintain the gaseous hydrogen in the gaseous chlorine under the standard for security.
(2) To have a construction of the top of the drying tower of gaseous chlorine and of the pipings not to cause the separation and accumulation of gaseous hydrogen.
(3) To provide earth-pieces made of tantalum, etc. with the tower and the pipings for gaseous chlorine.
(4) A elecctroconductive paint is painted on the whole surface of the outer wall of the tower for drying gaseous chlorine made of polyvinyl chloride and several copper hoops are installed on the thus painted surface with a suitable distance between them, and the tower is electrically grounded as a whole.
However, there is a possibility of the occurrence of unexpected accident of explosion due to the incomplete function of the determining apparatus by some troubles, and even if the concentration of hydrogen on the average in the whole process will be checked by the apparatus, a very much complicated and expensive system of determination is necessary for suppressing the partial and sudden raise of the concentration of hydrogen completely.
In addition, as will be clearly understood from Examples described later, sulfuric acid falling as droplets in the dried gaseous chlorine is highly charged with positive electricity, and the wall of the drying tower made of polyvinyl chloride is negatively charged.
Even by providing an electroconductive layer around the outer surface of the wall of the drying tower made of polyvinyl chloride for removing these electric charges, the effect of preventing the accidents of explosion cannot be expected.
Accordingly, an offer of the tower for drying gaseous chlorine, which is anti-corrosive against concentrated sulfuric acid and gaseous chlorine and is electroconductive not to accumulate static electricity has been demanded for removing the static charge within the tower to prevent the accident of explosion and to carry out the safe drying of gaseous chlorine.
As a result of their enthusiastic studies, the present inventors have found that by using a high polymeric composite material based on a resin selected from thermoplastic resins such as polypropyrene, polyvinyl chloride, etc. or thermosetting resins such as phenol resins, etc. as a matrix and containing 10 to 30% by weight of carbon fibers and 5 to 20% by weight of electro-conductive carbon black, that is, containing 15 to 50% by weight of total carbonaceous material as the wall material for the tower for drying gaseous chlorine and further by grounding the material, it is posible to effectively remove the static charge of the tower for drying gaseous chlorine generating from the electrolysis of sodium chloride.
Namely, the object of the present invention lies in offering a tower for drying gaseous chlorine for which a high polymeric composite material containing carbon fibers and electroconductive carbon black as the reinforcing material and the material for giving electroconductivity is used as a wall-forming material having a function of removing static charge which generates within the tower.